As cardiac stimulation devices become more technologically complex, it has become challenging to reduce their overall size or even maintain their existing size. Reducing the size of a cardiac stimulation device benefits the patient who must carry the device, and it may also lead to an improved procedure for implanting the device. Accordingly, there are continuous efforts within the medical device industry to reduce the size of pacemakers, defibrillators and other implantable medical devices.
Cardiac stimulation devices typically contain, among other components, an outer casing which houses an electronics module, a battery, various interconnection circuitry, and stimulus leads for attachment to a patient's heart. The electronics module of such cardiac stimulation devices is often a multi-level hybrid circuit structure. The multi-level module is ordinarily designed to achieve a low-volume configuration to facilitate placement within the limited confines of an associated housing. Multi-level circuit modules may contain separate vertically stacked substrates, i.e., platforms, having individual circuit components mounted on the substrates. A protective cover, or lid, is typically placed over any exposed electronic circuitry of the multi-level module.
Depending upon the particular design, multi-level modules found in cardiac stimulation devices, or other suitable devices, may require interconnection among components or terminals of different platform levels. The interconnection among components of different levels may be made through miniature wire-bonds attached to individual contacts. To achieve interconnection among various platform levels, it is common to design the various platforms of different size so that the wire bonds may extend over an edge of one platform in order to contact a surface of a second platform.
There have been many approaches documented in prior art publications for constructing a multi-layer or three-dimensional circuit module. For example, in U.S. Pat. No. 5,222,014 issued to Lin, and Japanese publication No. 1-147850 issued to Kuwabara, independent circuit platforms are stacked above a substrate and interconnected through solder joints or wire-bonds. In each case a lid may be placed over the structure to protect the underlying circuitry. In such prior art circuit structures, the interconnection among various platform levels is made by extending a wire-bond, or other connection means, over an edge of a higher platform to a lower platform level. One drawback to this method of interconnection is the need for platform levels of successively smaller surface area. For example, in Japanese publication No. 62-260353 issued to Yoshida, individual platform levels are placed on top of one another for achieving a stacked semiconductor device. However, because each stacked platform of the device in Yoshida decreases in size, a successively smaller surface area is available on each of the stacked platforms.
A second drawback of the device in Yoshida, and other devices found in the prior art, is the requirement that all multi-level connections be made about an outside perimeter of a particular platform level. This requirement limits the design constraints with respect to component layout which in some cases may lead to a less than optimal configuration of components atop the platform levels.
Therefore, there is a need in the art for an improved pacemaker design which allows for the packaging of internal electrical components to optimize use of multi-level surface area. There is also a need in the art for an improved multi-level circuit structure module which allows for greater flexibility in the design of electrical component layout.